There conventionally is a known semiconductor light emitting device for emitting white-based light that is composed of a combination of a near UV LED (strictly speaking, a near UV LED chip) having an emission peak in a wavelength region of near UV exceeding 350 nm and not more than 410 nm and a phosphor layer including a plurality of inorganic phosphors each emitting fluorescence having an emission peak in a visible light wavelength range not less than 380 nm and not more than 780 nm. The semiconductor light emitting device using the inorganic phosphors is widely used because it has higher durability than a semiconductor light emitting device using organic phosphors.
It is noted that light having a luminescent chromaticity point (x, y) in the CIE chromaticity diagram of 0.21≦x≦0.48 and 0.19≦y≦0.45 is herein defined as white-based light.
Such known semiconductor light emitting devices are disclosed in, for example, Japanese Laid-Open Patent Publication Nos. 11-246857, 2000-183408, 2000-509912 and 2001-143869.
Japanese Laid-Open Patent Publication No. 11-246857 describes a semiconductor light emitting device using, as a red phosphor, a lanthanum oxysulfide phosphor represented by a general formula, (La1-x-yEuxSmy)2O2S (wherein 0.01≦x≦0.15 and 0.0001≦y≦0.03) to be combined with a near UV LED for emitting light of a wavelength of approximately 370 nm having a light emitting layer made of a gallium nitride-based compound semiconductor. Also, Japanese Laid-Open Patent Publication No. 11-246857 discloses a technique related to a semiconductor light emitting device for emitting white-based light with an arbitrary color temperature composed of an appropriate combination of the red phosphor and other blue and green phosphors.
Japanese Laid-Open Patent Publication No. 2000-183408 describes a semiconductor light emitting device composed of a UV LED chip having a light emitting layer made of a gallium nitride-based compound semiconductor for emitting UV with an emission peak in the vicinity of 370 nm, a first phosphor layer including a blue phosphor for absorbing the UV and emitting blue light and a second phosphor layer including a yellow/orange phosphor for absorbing the blue light and emitting yellow/orange light. As the blue phosphor, at least one kind of blue phosphor selected from the following (1) through (3) is used:
(1) A bivalent europium-activated halophosphate phosphor substantially represented by a general formula, (M1, Eu)10(PO4)6Cl2 (wherein M1 is at least one element selected from the group consisting of Mg, Ca, Sr and Ba);
(2) a bivalent europium-activated aluminate phosphor substantially represented by a general formula, a(M2, Eu)O-bAl2O3 (wherein M2 is at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Zn, Li, Rb and Cs, and a and b are numerical values satisfying a>0, b>0 and 0.2≦a/b≦1.5; and
(3) a bivalent europium/manganese-activated aluminate phosphor substantially represented by a general formula, a(M2, Euv, Mnw)O-bAl2O3 (wherein M2 is at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Zn, Li, Rb and Cs, and a, b, v and w are numerical values satisfying a>0, b>0, 0.2≦a/b≦1.5 and 0.001≦w/v≦0.6.
Also, as the yellow/orange phosphor, a trivalent cerium-activated aluminate phosphor substantially represented by a general formula, (Y1-x-yGdxCey)3Al5O12 (wherein x and y are numerical values satisfying 0.1≦x≦0.55 and 0.01≦y≦0.4) (hereinafter referred to as a YAG-based phosphor) is used.
Furthermore, Japanese Laid-Open Patent Publication No. 2000-509912 discloses a semiconductor light emitting device composed of a combination of a UV LED having an emission peak in a wavelength region not less than 300 nm and not more than 370 nm, a blue phosphor having an emission peak in a wavelength region not less than 430 nm and not more than 490 nm, a green phosphor having an emission peak in a wavelength region not less than 520 nm and not more than 570 nm, and a red phosphor having an emission peak in a wavelength region not less than 590 nm and not more than 630 nm. In this semiconductor light emitting device, BaMgAl10O10O17:Eu, Sr5(PO4)3Cl:Eu or ZnS:Ag (both having an emission peak wavelength of 450 nm) is used as the blue phosphor, ZnS:Cu (having an emission peak wavelength of 550 nm) or BaMgAl10O17:Eu,Mn (having an emission peak wavelength of 515 nm) is used as the green phosphor, and Y2O2S:Eu3+ (having an emission peak wavelength of 628 nm), YVO4:Eu3+ (having an emission peak wavelength of 620 nm), Y(V, P, B)O4:Eu3+ (having an emission peak wavelength of 615 nm), YNbO4:Eu3+ (having an emission peak wavelength of 615 nm), YTaO4:Eu3+ (having an emission peak wavelength of 615 nm), or [Eu(acac)3(phen)] (having an emission peak wavelength of 611 nm) is used as the red phosphor.
On the other hand, Japanese Laid-Open Patent Publication No. 2001-1438769 discloses a semiconductor light emitting device composed of a combination of an organic LED including an organic material as a light emitting layer and having an emission peak in a wavelength region ranging between blue violet and near UV of 430 nm or less, or an inorganic LED including an inorganic material as a light emitting layer and having an emission peak in the wavelength region ranging between blue violet and near UV, and a blue phosphor, a green phosphor and a red phosphor. In this semiconductor light emitting device, Sr2P2O7:Sn4+, Sr4Al14O25:Eu2+, BaMgAl10O17:Eu2+, SrGa2S4:Ce3+, CaGa2S4:Ce3+, (Ba, Sr)(Mg, Mn)Al10O17:Eu2+, (Sr, Ca, Ba, Mg)10(PO4)6Cl2:Eu2+, BaAl2SiO8:Eu2+, Sr2P2O7:Eu2+, Sr5(PO4)3Cl:Eu2+, (Sr, Ca, Ba)5(PO4)3Cl:Eu2+, BaMg2Al16O27:Eu2+, (Ba, Ca)5(PO4)3Cl:Eu2+, Ba3MgSi2O8:Eu2+ or Sr3MgSi2O8:Eu2+ is used as the blue phosphor, (BaMg)Al16O27:Eu2+,Mn2+, Sr4Al14O25:Eu2+, (SrBa)Al2Si2O8:Eu2+, (BaMg)2SiO4:Eu2+, Y2SiO5:Ce3+,Tb3+, Sr2P2O7—Sr2B2O7:Eu2+, (BaCaMg)5(PO4)3Cl:Eu2+, Sr2Si3O82SrCl2:Eu2+, Zr2SiO4—MgAl11O19:Ce3+,Tb3+, Ba2SiO4:Eu2+, Sr2SiO4:Eu2+ or (BaSr)SiO4:Eu2+ is used as the green phosphor, and Y2O2S:Eu2+, YAlO3:Eu3+, Ca2Y2(SiO4)6:Eu3+, LiY9(SiO4)6O2:Eu3+, YVO4:Eu3+, Gd2O3:Eu3+, Gd2O2S:Eu3+ or Y(P, V)O4:Eu3+ is used as the red phosphor.
In this manner, in a conventional semiconductor light emitting device for emitting white-based light, the white-based light is obtained through color mixture of light emitted by a blue-based phosphor, a green-based phosphor and a red-based phosphor or light emitted by a blue-based phosphor and a yellow-based phosphor.
In the conventional semiconductor light emitting device employing the method in which the white-based light is obtained through the color mixture of the light emitted by the blue-based phosphor and the yellow-based phosphor, the aforementioned YAG-based phosphor is used as the yellow-based phosphor. Also, the YAG-based phosphor minimally emits light through excitation by light of a wavelength region exceeding 350 nm and not more than 400 nm, particularly by near UV of a wavelength not more than 360 nm and not less than 400 nm emitted by a near UV LED having a light emitting layer made of a gallium nitride-based compound semiconductor, and highly efficiently emits yellow light through excitation by a blue-based light of a wavelength not less than 400 nm and not more than 530 nm. Therefore, in the conventional semiconductor light emitting device using the YAG-based phosphor, the blue-based phosphor is indispensable for obtaining the white-based light so that a yellow-based phosphor can be excited by blue light emitted by the blue-based phosphor.
Such a semiconductor light emitting device for emitting white-based light is known to be much in demand for light emitting apparatuses such as an illumination apparatus and a display apparatus.
On the other hand, a semiconductor light emitting device composed of a combination of an LED and an inorganic compound phosphor other than the YAG-based phosphor is conventionally known. The aforementioned Japanese Laid-Open Patent Publication No. 2001-143869 describes a semiconductor light emitting device using a Ba2SiO4:Eu2+, Sr2SiO4:Eu2+, Mg2SiO4:Eu2+, (BaSr)2SiO4:Eu2+ or (BaMg)2SiO4:Eu2+ silicate phosphor.
However, in the semiconductor light emitting device described in this Japanese Laid-Open Patent Publication No. 2001-143869, any silicate phosphor is applied as a green-based phosphor not as a yellow-based phosphor. Also, it is regarded that an organic LED is more preferably used than an inorganic LED made of an inorganic compound from the viewpoint of luminous efficiency. Specifically, the invention disclosed in this publication does not relate to a semiconductor light emitting device composed of a combination of a near UV LED and blue-based, green-based, yellow-based and red-based phosphors but relates to a semiconductor light emitting device composed of a combination of a near UV LED, and preferably an organic LED, and three kinds of inorganic compounds of blue-based, green-based and red-based phosphors.
According to experiments carried out by the present inventors, the Sr2SiO4:Eu2+ silicate phosphor described in this Japanese Laid-Open Patent Publication No. 2001-143869 is a phosphor that may have two crystalline phases (that is, the orthorhombic system and the monoclinic system), and at least within a practical range of an Eu2+ luminescent center content (=the number of Eu atoms/(the number of Sr atoms+the number of Eu atoms): x) of 0.01≦x≦0.05, the orthorhombic Sr2SiO4:Eu2+(α′-Sr2SiO4:Eu2+) is a yellow-based phosphor emitting yellow light having an emission peak in the vicinity of a wavelength of 560 through 575 nm, and the monoclinic Sr2SiO4:Eu2+(β-Sr2SiO4:Eu2+) is a green-based phosphor emitting green light having an emission peak in the vicinity of a wavelength of 545 nm. Accordingly, the Sr2SiO4:Eu2+ green phosphor described in Japanese Laid-Open Patent Publication No. 2001-143869 can be regarded as the monoclinic Sr2SiO4:Eu2+ phosphor.
At this point, the silicate phosphor will be described. A silicate phosphor represented by a chemical formula, (Sr1-a3-b3-xBaa3Cab3Eux)2SiO4 (wherein a3, b3 and x are numerical values satisfying 0≦a3≦1, 0≦b3≦1, 0<x<1) is conventionally known. This silicate phosphor is a phosphor having been examined as a phosphor for use in a fluorescent lamp, and is known to be a phosphor whose emission peak wavelength is changed in a range approximately not less than 505 nm and not more than 598 nm by changing the composition of Ba—Sr—Ca. Furthermore, it is known to be a phosphor that exhibits comparatively high luminous efficiency in illumination ranging between 170 nm and 350 nm (See J. Electrochemical Soc. Vol. 115, No. 11 (1968) pp. 1181–1184).
This document, however, does not describe that the silicate phosphor exhibits high luminous efficiency through excitation by near UV of a long wavelength region exceeding 350 nm. Therefore, it has not been known that the silicate phosphor is a phosphor that emits yellow-based light of a wavelength not less than 550 nm and less than 600 nm with high efficiency through the excitation by light of the aforementioned near UV wavelength region exceeding 350 nm and not more than 410 nm, and particularly, by near UV of a wavelength in the vicinity of 370 through 390 nm emitted by a near UV LED having a light emitting layer made of a gallium nitride-based compound semiconductor.
In the conventional semiconductor light emitting device and light emitting apparatus composed of a combination of a near UV LED and a phosphor layer including a plurality of phosphors, the semiconductor light emitting device and light emitting apparatus are constructed by employing the method in which white-based light is obtained through the color mixture of light emitted by a blue-based phosphor, a green-based phosphor and a red-based phosphor or light emitted by a blue-based phosphor and a yellow-based phosphor.
It is noted that any of various display apparatuses (such as an LED information display terminal, an LED traffic light, and an LED stop lamp and an LED direction indicator lamp of a vehicle) and various illumination apparatuses (such as an LED indoor or outdoor lighting, a car LED lamp, an LED emergency light and an LED surface emitting light source) is herein widely defined as a light emitting apparatus.
In the conventional semiconductor white-based light emitting device and semiconductor white-based light emitting apparatus composed of the combination of a near UV LED and a phosphor layer including a plurality of phosphors, the luminous flux of the white-based light emitting by the semiconductor light emitting device or the semiconductor light emitting apparatus is low. This is derived from the following: Since a phosphor capable of exhibiting high luminous efficiency through excitation by near UV of a wavelength exceeding 350 nm and not more than 410 nm has not been sufficiently developed, with respect to all of blue-based phosphors, green-based phosphors and red-based phosphors, the number of kinds of phosphors usable in the semiconductor white-based light emitting device or the semiconductor white-based light emitting apparatus is small. Therefore, the number of blue-based, green-based and red-based phosphors exhibiting comparatively high luminous efficiency is limited to be small as well as the shape of an emission spectrum of the resultant white-based light is limited. Furthermore, this is also because the white-based light is obtained through the color mixture of light emitted by the three kinds of blue-based, green-based and red-based phosphors or light emitted by the two kinds of blue-based and yellow-based phosphors.
In order to obtain white-based light with high luminous flux and a large average number of color rendering Ra (of 70 or more) through the color mixture of light emitted by three kinds of blue-based, green-based and red-based phosphors, all of the blue-based phosphor, the green-based phosphor and the red-based phosphor should have high luminous efficiency, and if any one of these phosphors is a phosphor with low luminous efficiency, the luminous flux of the resultant white-based light is lowered because of the color balance of the white-based light.